PLANAR DUAL-GATE FIELD EFFECT TRANSISTORS (FETs)

ABSTRACT

A semiconductor structure and the associated method for fabricating the same. The semiconductor structure includes (a) a semiconductor substrate, (b) a back gate region on the semiconductor substrate, (c) a back gate dielectric region on the back gate region, (d) a semiconductor region on the back gate dielectric region comprising a channel region disposed between first and second source/drain (S/D) regions, (e) a main gate dielectric region on the semiconductor region, (f) a main gate region on the main gate dielectric region, (g) a first contact pad adjacent to the first S/D region and electrically insulated from the back gate region, and (h) a first buried dielectric region that physically and electrically isolates the first contact pad and the back gate region, and wherein the first buried dielectric region has a first thickness in the first direction at least 1.5 times a second thickness of the back gate region.

This application is a continuation application claiming priority to Ser. No. 10/907,745, filed Apr. 14, 2005.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to field effect transistors (FETs), and more specifically, to planar dual-gate FETs (i.e., planar FETs that have main gates and back gates).

2. Related Art

The switching speed of a typical planar dual-gate FET depends on, among other things, the capacitances between the S/D regions and the back gate region of the typical planar dual-gate FET. The higher such capacitances, the longer it takes for switching (because it takes time for charging and discharging the capacitors), which is undesirable.

Therefore, there is a need for a planar dual-gate FET (and a method for fabricating the same) in which capacitances between the S/D regions and the back gate region are relatively lower than that of the prior art.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor structure, comprising (a) a semiconductor substrate; (b) a back gate region on the semiconductor substrate; (c) a back gate dielectric region on the back gate region in a first direction, wherein the first direction is perpendicular to an interfacing surface between the back gate region and the semiconductor substrate; (d) a semiconductor region on the back gate dielectric region in the first direction, wherein the semiconductor region comprises a channel region and first and second source/drain (S/D) regions, wherein the channel region is disposed between the first and second S/D regions, and wherein the semiconductor region is electrically insulated from the back gate region by the back gate dielectric region; (e) a main gate dielectric region on the semiconductor region in the first direction; (f) a main gate region on the main gate dielectric region in the first direction, wherein the semiconductor region is electrically insulated from the main gate region by the main gate dielectric region; and (g) a first contact pad adjacent to the first S/D region in a second direction and in direct physical contact with the first S/D region, wherein the second direction is perpendicular to the first direction and pointing from the second S/D region to the first S/D region; and (h) a first buried dielectric region directly beneath the first contact pad in the first direction and in direct physical contact with the first contact pad and the back gate region, wherein the first buried dielectric region physically and electrically isolates the first contact pad and the back gate region, and wherein the first buried dielectric region has a first thickness in the first direction at least 1.5 times a second thickness of the back gate region.

The present invention also provides a semiconductor structure fabrication method, comprising providing a semiconductor substrate, a back gate layer on the semiconductor substrate, a back gate dielectric layer on the back gate layer in a first direction, wherein the first direction is perpendicular to an interfacing surface between the back gate layer and the semiconductor substrate, a semiconductor layer on the back gate dielectric layer in the first direction, wherein the semiconductor layer is electrically insulated from the back gate layer by the back gate dielectric layer, and a main gate dielectric layer on the semiconductor layer in the first direction; removing portions of the main gate dielectric layer, the semiconductor layer, the back gate dielectric layer, and the back gate layer so as to form first and second trenches such that a back gate dielectric region disposed between the first and second trenches results from the back gate dielectric layer, a semiconductor region disposed between the first and second trenches results from the semiconductor layer, and a main gate dielectric region disposed between the first and second trenches results from the main gate dielectric layer; forming first and second buried dielectric regions in the first and second trenches, respectively, and in direct physical contact with the back gate dielectric layer, wherein the first buried dielectric region has a first thickness in the first direction at least 1.5 times a second thickness of the back gate region; and forming first and second contact pads on the first and second buried dielectric regions and in the first and second trenches, respectively, wherein the first and second contact pads are in direct physical contact with the semiconductor region, wherein the first and second contact pads are electrically insulated from the back gate region

The present invention also provides a planar dual-gate FET (and a method for fabricating the same) in which capacitances between the S/D regions and the back gate region are relatively lower than that of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 illustrate cross-section views of a semiconductor structure going through different fabrication steps, in accordance with embodiments of the present invention.

FIGS. 6A-6D illustrate a process for forming the semiconductor structure of FIG. 1, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1-5 illustrate cross-section views of a semiconductor structure 100 going through different fabrication steps, in accordance with embodiments of the present invention. More specifically, with reference to FIG. 1, the fabrication steps can start out with a structure 100 comprising (a) a semiconductor (e.g., silicon, germanium, etc.) substrate 110, (b) a buried oxide (BOX) layer 120 on top of the semiconductor substrate 110, (c) a back gate layer 130 on top of the BOX layer 120, (d) a back gate dielectric layer 140 on top of the back gate layer 130, and (e) a semiconductor (i.e., silicon, germanium, etc.) layer 150 on top of the back gate dielectric layer 140. In one embodiment, the BOX layer 120 can comprise silicon dioxide, the back gate layer 130 can comprise silicon, and the back gate dielectric layer 140 can comprise silicon dioxide.

In one embodiment, the structure 100 of FIG. 1 can be formed using a double SOI (silicon on insulator) process. FIGS. 6A-6D illustrate the formation of the structure 100 of FIG. 1 using the double SOI process. With reference to FIG. 6A, the double SOI process can start with a silicon substrate 610.

Next, the BOX layer 120 can be formed on top of the silicon substrate 610 by, illustratively, chemical vapor deposition (CVD).

Next, hydrogen II ions can be implanted in the silicon substrate 610 so as to form a hydrogen ion layer 612 embedded in the silicon substrate 610, resulting in the structure 100 of FIG. 6A. The portion of the silicon substrate 610 above the hydrogen II ion layer 612 will become the back gate layer 130.

Next, with reference to FIG. 6B, the semiconductor substrate 110 can be bonded to BOX layer 120 resulting in the structure 100 of FIG. 6B.

Next, the structure 100 of FIG. 6B can be annealed so that the structure 100 of FIG. 6B splits along the hydrogen ion layer 612. The upper portion of the structure 100 of FIG. 6B after the split is turned upside down resulting in the structure 100 of FIG. 6C.

Next, a structure similar to the structure 100 of FIG. 6A can be turned upside down and bonded to the semiconductor layer 130 of FIG. 6C resulting in the structure 100 of FIG. 6D.

Next, the structure 100 of FIG. 6D can be annealed so that the structure 100 of FIG. 6D splits along the hydrogen ion layer 622. The lower portion of the structure 100 of FIG. 6D after the split can be used as the structure 100 of FIG. 1.

Next, with reference to FIG. 2, shallow trench isolation (STI) regions 160 a and 160 b can be formed in the semiconductor layer 150 using any conventional method.

Next, an oxide layer 210 can be formed on top of the entire structure 100 of FIG. 1 by, illustratively, thermal oxidation or chemical vapor deposition (CVD). Next, a nitride layer 220 can be formed on top of the oxide layer 210 by, illustratively, CVD.

Next, with reference to FIG. 3, two trenches 310 a and 310 b can be formed in the structure 100 of FIG. 2. More specifically, in one embodiment, the trenches 310 a and 310 b can be formed by directionally etching (e.g., using reactive ion etching or RIE etch) in turn the nitride layer 220, the oxide layer 210, the semiconductor layer 150, the back gate dielectric layer 140, the back gate layer 130, and the buried oxide layer 120 in that order. In one embodiment, the directional etch that forms the trenches 310 a and 310 b etches completely through the back gate layer 130 but does not etch completely through the buried oxide layer 120. It should be noted that portions of the nitride layer 220, the oxide layer 210, the semiconductor layer 150, the back gate dielectric layer 140, the back gate layer 130 that are disposed between the trenches 310 a and 310 b can be referred to as the nitride region 220′, the oxide region 210′, the semiconductor region 150′, the back gate dielectric region 140′, and the back gate region 130′, respectively.

Next, buried dielectric regions 320 a and 320 b can be formed in the trenches 310 a and 310 b, respectively. In one embodiment, the buried dielectric regions 320 a and 320 b can comprise silicon dioxide (SiO₂), and can be formed by, illustratively, (a) depositing a SiO₂ layer (not shown) on top of the entire structure 100 of FIG. 3 such that the trenches 310 a and 310 b are completely filled with SiO₂ material, then (b) etching back the deposited SiO₂ layer until top surfaces 322 a and 322 b of the buried dielectric regions 320 a and 320 b, respectively, are at the level of the semiconductor region 150′. The process of forming the buried dielectric regions 320 a and 320 b by filling the trenches 310 a and 310 b and etching back can be referred to as a fill and recess process.

Next, with reference to FIG. 4, contact pads 410 a and 410 b can be formed on top of the buried dielectric regions 320 a and 320 b in the trenches 310 a and 310 b (FIG. 3), respectively. In one embodiment, the contact pads 410 a and 410 b can comprise polysilicon. The polysilicon contact pads 410 a and 410 b can be formed by (a) depositing polysilicon on the entire structure 100 of FIG. 3 and then (b) planarizing using a conventional chemical mechanical polishing (CMP) process until a top surface 212 of the oxide region 210′ is exposed to the atmosphere. The process of forming the contact pads 410 a and 410 b by depositing polysilicon and then planarizing can be referred to as a deposit and planarize process. Because the top surfaces 322 a and 322 b of the buried dielectric regions 320 a and 320 b, respectively, are at the level of the semiconductor region 150′, the contact pads 410 a and 410 b are in direct physical contact with the semiconductor region 150′. Next, the oxide layer 210 (including the oxide region 210′) is removed.

Next, with reference to FIG. 5, a main gate stack 510,505 comprising a main gate dielectric region 505 and a main gate region 510 can be formed on top of the semiconductor region 150′. In one embodiment, the main gate region 510 can comprise polysilicon. The main gate stack 510,505 can be formed by (a) depositing a main gate dielectric layer (not shown) on top of the entire structure 100 of FIG. 4 (after the oxide layer 210 is removed), then (b) depositing a main gate layer (not shown) on top of the main gate dielectric layer, and then (c) etching back the deposited main gate layer and the main gate dielectric layer to form the main gate stack 510,505 using any conventional photolithography process.

Next, main gate spacers 512 a and 512 b can be formed on side walls of the main gate stack 510,505. The main gate spacers 512 a and 512 b can comprise silicon dioxide and can be formed using any conventional method.

Next, the main gate stack 510,505 and the main gate spacers 512 a and 512 b can be used as a mask to dope the semiconductor region 150′ so as to form source/drain (S/D) regions 520 a and 520 b in the semiconductor region 150′. In one embodiment, ion implantation can be used for this doping process. The portion 530 of the semiconductor region 150′ directly beneath the gate stack 510, 505 and disposed between the S/D regions 520 a and 520 b can be referred to as the channel region 530.

Next, a silicide region 540 a can be formed on top and in direct physical contact with both the contact pad 410 a and the S/D region 520 a, while a silicide region 540 b can be formed on top and in direct physical contact with both the contact pad 410 b and the S/D region 520 b.

The silicide regions 540 a and 540 b can be formed by (a) depositing a metal material (e.g., cobalt, titanium, etc.) on top of the entire structure 100 of FIG. 5 (without the metal vias 550 a, 550 b, and 550 c, and the silicide regions 540 a and 540 b at this time), then (b) heating up the metal material such that the metal material chemically reacts with silicon of the contact pads 410 a and 410 b and the S/D regions 520 a and 520 b to form the silicide regions 540 a and 540 b, and then (c) removing the remaining metal material by a wet etch step.

Next, a dielectric layer 545 can be formed on top of the entire structure 100 by, illustratively, CVD. Next, metal vias 550 a and 550 b can be formed in the dielectric layer 545 and on top of the silicide regions 540 a and 540 b, respectively, while a metal via 550 c can be formed in the dielectric layer 545 and on top of the main gate region 510. The vias 550 a, 550 b, 550 c can be used to electrically couple the structure 100 to an upper interconnect layer (not shown).

It should be understood that, although not shown, a contact to the back gate region 130′ may be formed, using any conventional method, to provide an electric connection between an upper interconnect layer (not shown) and the back gate region 130′.

In summary, because of the buried dielectric region 320 a, the first S/D block (including the S/D region 520 a, the contact pad 410 a, and silicide region 540 a, and the metal via 550 a) forms with the back gate region 130′ a first capacitance relatively lower than that of the prior art. Similarly, because of the buried dielectric region 320 b, the second S/D block (including the S/D region 520 b, the contact pad 410 b, the silicide region 540 b, and the metal via 550 b) forms with the back gate region 130′ a second capacitance relatively lower than that of the prior art. As a result, the structure 100 can switch faster than devices of the prior art.

While particular embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention. 

1. A semiconductor structure, comprising: (a) a semiconductor substrate; (b) a back gate region on the semiconductor substrate; (c) a back gate dielectric region on the back gate region in a first direction, wherein the first direction is perpendicular to an interfacing surface between the back gate region and the semiconductor substrate; (d) a semiconductor region on the back gate dielectric region in the first direction, wherein the semiconductor region comprises a channel region and first and second source/drain (S/D) regions, wherein the channel region is disposed between the first and second S/D regions, and wherein the semiconductor region is electrically insulated from the back gate region by the back gate dielectric region; (e) a main gate dielectric region on the semiconductor region in the first direction; (f) a main gate region on the main gate dielectric region in the first direction, wherein the semiconductor region is electrically insulated from the main gate region by the main gate dielectric region; and (g) a first contact pad adjacent to the first S/D region in a second direction and in direct physical contact with the first S/D region, wherein the second direction is perpendicular to the first direction and pointing from the second S/D region to the first S/D region; and (h) a first buried dielectric region directly beneath the first contact pad in the first direction and in direct physical contact with the first contact pad and the back gate region, wherein the first buried dielectric region physically and electrically isolates the first contact pad and the back gate region, and wherein the first buried dielectric region has a first thickness in the first direction at least 1.5 times a second thickness of the back gate region.
 2. The structure of claim 1, wherein the first buried dielectric region is in direct physical contact with the first S/D region, the back gate dielectric region, and the back gate region.
 3. The structure of claim 1, wherein the back gate dielectric region and the first buried dielectric region comprise an oxide material.
 4. The structure of claim 1, wherein no portion of the back gate region is directly beneath the first buried dielectric region in the first direction.
 5. A semiconductor structure fabrication method, comprising: providing a semiconductor substrate, a back gate layer on the semiconductor substrate, a back gate dielectric layer on the back gate layer in a first direction, wherein the first direction is perpendicular to an interfacing surface between the back gate layer and the semiconductor substrate, a semiconductor layer on the back gate dielectric layer in the first direction, wherein the semiconductor layer is electrically insulated from the back gate layer by the back gate dielectric layer, and a main gate dielectric layer on the semiconductor layer in the first direction; removing portions of the main gate dielectric layer, the semiconductor layer, the back gate dielectric layer, and the back gate layer so as to form first and second trenches such that a back gate dielectric region disposed between the first and second trenches results from the back gate dielectric layer, a semiconductor region disposed between the first and second trenches results from the semiconductor layer, and a main gate dielectric region disposed between the first and second trenches results from the main gate dielectric layer; forming first and second buried dielectric regions in the first and second trenches, respectively, and in direct physical contact with the back gate dielectric layer, wherein the first buried dielectric region has a first thickness in the first direction at least 1.5 times a second thickness of the back gate region; and forming first and second contact pads on the first and second buried dielectric regions and in the first and second trenches, respectively, wherein the first and second contact pads are in direct physical contact with the semiconductor region, wherein the first and second contact pads are electrically insulated from the back gate region.
 6. The method of claim 5, wherein the second buried dielectric region has a third thickness in the first direction at least 1.5 times a fourth thickness of the back gate region.
 7. The method of claim 5, further comprising forming a main gate region directly on the main gate dielectric region, wherein the main gate region is electrically insulated from the semiconductor region by the main gate dielectric region.
 8. The method of claim 7, further comprising forming first and second source/drain regions in the semiconductor region such that a channel region of the semiconductor region directly beneath the main gate region in the first direction is disposed between the first and second source/drain regions, wherein the first and second source/drain regions are in direct physical contact with the first and second contact pads, respectively.
 9. The method of claim 5, wherein said removing portions comprises directionally etching the main gate dielectric layer, the semiconductor layer, the back gate dielectric layer, and the back gate layer in that order and in the first direction.
 10. The method of claim 5, wherein said forming the first and second buried dielectric regions in the first and second trenches, respectively, is performed by a fill and recess process.
 11. The method of claim 5, wherein said forming the first and second contact pads is performed by a deposit and planarize process.
 12. The method of claim 5, further comprising forming a first silicide region on the first contact pad and the semiconductor region in the first direction and in direct physical contact with the first contact pad and the semiconductor region.
 13. The method of claim 12, further comprising forming a first metal via on the first silicide region in the first direction and in direct physical contact with the first silicide region.
 14. The method of claim 13, further comprising: forming a second silicide region on the second contact pad and the semiconductor region in the first direction and in direct physical contact with the second contact pad and the semiconductor region; and forming a second metal via on the second silicide region in the first direction and in direct physical contact with the second silicide region. 